Line of Duty, season four on BBC One

26 Mar 17 – 31 May 17, Line of Duty starts at 9pm, Sunday 26 March on BBC1

Line of Duty concludes this Sunday with an explosive finale. What happens when the police investigate themselves?

Line of Duty, season four, BBC One

Line of Duty, season four on BBC One Line of Duty, season four on BBC One Helena Kealey

She only has one arm, she's arrested her husband for the crime she committed, and AC-12 are on to her: it's the final episode of season four of Line of Duty and what everyone wants to know is: will DCI Roz Huntley get away with the murder of Tim Ifield? Don't worry, we're not going to tell you: this is a spoiler-free review of the series.

Season four began with the murder by Huntley (Thandie Newton), before rejoining the investigations of AC-12, a police anti-corruption unit operating within the West Midlands force. The series swaps the usual cop narrative of goodies versus baddies for a deliciously morally ambiguous minefield. In Line of Duty right and wrong are indistinct and blurred, the truth re-buried as soon as it is dug up.

AC-12 officers Ted Hastings (played by Adrian Dunbar), Kate Fleming (Vicky McClure) and Steve Arnott (Martin Compston) have to return to save the day and, this time, investigate Roz Huntley (Thandie Newton), a senior detective looking into the abduction of a young woman.

Forensic Investigator Tim Ifield (played by Jason Watkins), BBC One Line of Duty

Newton is unquestionably the star of the show. Hot off the set of Westworld – and for a while tipped by some to be the next Dr Who before Kris Marshall claimed that crown – she gives a compelling performance as a ballsy, determined detective out to prove her worth, and the guilt of the man she arrested.

Huntley is obstructed by forensics expert Tim Ifield (Jason Watkins), who believes that mitigating evidence, which would prove the innocence of the man arrested, has been intentionally overlooked by an overly zealous detective with an inexplicable agenda of her own.

'There's facts. And then there's the truth,' observes Hastings. Despite being written long before the rise of Trump and fake news, creator Jed Mercurio manages to capture the ambiguous nature of truth, as forensic evidence is spun into wildly different narratives – a counter-factual conundrum especially poignant in the light of 2016's political events.

And of course, the series ends with an unexpected twist. Doesn't it always?